1. Field of the Invention
The present invention relates to a fixing device which is employed in, e.g., a copying machine or printer apparatus using toner, and fixes a toner image onto a medium to be transferred.
2. Description of the Related Art
A fixing device which is assembled into a copying machine using an electrophotographic process heats and melts toner on a medium to be transferred, and fixes the toner onto the medium.
As a toner heating method usable in the fixing device, a method using radiation heat obtained by turning on a filament lamp, and a flash-fixing method using a flash lamp have widely been known. Recently, a fixing device using an induction heating device as a heat generation source is put into practical use.
In many cases, the fixing device uses a heat (fixing) roller which incorporates a heater, and a press roller which is pressed against the heat roller at one point on the outer surface of the heat roller at a predetermined pressure. With this structure, heat can be efficiently supplied from the heat generation source to toner. A pressure for fixing melted toner onto a medium to be transferred can be easily applied to the medium and toner.
Most of toners used in a copying machine and printer apparatus are granular particles or powder particles prepared by coating a pigment or dye with a hot-melt resin. Toner which is not fixed to a medium to be transferred often remains at a portion of the fixing device which contacts melted toner. Thus, many fixing devices having a cleaning device which recovers toner left on the heat roller are commercially available.
As a method of cleaning the heat roller, felt is brought into contact with the outer surface of the heat roller, and oil which prevents toner from attaching to the heat roller is supplied. Toner is fixed to a portion where it is brought into contact with the heat roller in the use of felt. Thus, the felt is formed into a roller shape to prevent toner from being fixed to one portion.
Even with the felt roller, the toner catchable amount decreases and the image quality degrades during the maintenance cycle because of an increase in the image forming speed of a copying machine and printer apparatus and prolongation of the maintenance cycle of the copying machine and printer apparatus. If felt containing a large amount of toner is pressed against the heat roller for a long time, the heat roller surface is scratched.
To solve these problems, a web cleaning method of changing (shifting) the position of felt brought into contact with the heat roller in accordance with the cumulative image forming count (lapse of a predetermined time).
In the use of the felt web, the web containing toner is sequentially taken up. The replacement time of the cleaning device can be set by setting the web amount in accordance with the maintenance cycle. The web may be coated with oil which prevents toner from attaching to the heat roller.
Even if the felt web is adopted, the web amount assembled into the fixing device has an end portion (roll length) due to the maintenance cycle, the internal space of the apparatus, and the like.
Hence, the web must be replaced at the end of the web of the cleaning mechanism within the total operatable time (apparatus life) which is set in advance in the image forming apparatus.
Immediately after the web of the cleaning mechanism is replaced, an unnecessary web portion which is not taken up by the web-taking unit may be caught between the heat roller or belt of the fixing device and the press roller.
If the web is caught between the heat roller (belt) and the press roller, the heat roller surface is scratched and cannot be used (must be replaced) in the worst case. Replacement of the heat roller requires a long time and high cost.
When an unnecessary web portion is caught between the heat roller (belt) and the press roller although the heat roller need not be replaced, the web scrapes toner which is transferred and electrostatically attached to a sheet to be output in subsequent image formation. As a result, normal image formation may fail.
Even if the untaken web is not caught between the heat roller (belt) and the press roller, the web may not always be normally conveyed every predetermined number of image forming operations at a cleaning position where the web is in contact with the heat roller until an unnecessary web portion is completely taken up by the recovery side and the web normally functions. In this case, toner attached to the web may damage the heat roller surface.
Even when the heat roller surface is not damaged, the heat roller surface is not always satisfactorily cleaned, and an output sheet (printout) may be undesirably contaminated with toner. In a fixing device in which the heat roller is coated with oil serving as a release agent by using the web of the cleaning mechanism, the oil amount becomes unstable immediately after the web is replaced.